Light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent and fluorescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. One advantage of LEDs is that it is relatively easy to vary the light output intensity over a wide range, thus meeting the needs of a wide range of customers and/or applications using one or a relatively few number of LED components.
LEDs are inherently current-controlled devices, with the light output intensity varying with the applied current although in many applications a constant or variable voltage supply is used to power LEDs. In such scenarios, it is common to provide current regulation or current-limiting means to ensure that the LEDs operate at a desired and relatively constant current (and thus at a desired and relatively constant light intensity) and are not subject to over-current conditions which may damage the LEDs.
Various means for controlling or regulating the current have been employed. One simple approach is to use a resistor in series with an LED, as shown in FIG. 1A. The circuit of FIG. 1A includes power bus providing a constant voltage to power conductors 110 and 120. LEDs 130 are electrically coupled in series with current-limiting resistor 160 between power conductors 120 and 110. As is well known in the field, the disadvantage of this approach is while it does provide a measure of current-limiting functionality, the current through the LED varies proportionally with the applied voltage in accordance with Ohm's law.
Many different active circuits have been utilized to provide a relatively constant current over a relatively wide range of applied voltages. Various examples of such circuits or current control elements (CEs) are described in U.S. patent application Ser. No. 13/799,807, filed on Mar. 13, 2013 (“the '807 application”), and U.S. patent application Ser. No. 13/970,027, filed on Aug. 19, 2013 (“the '027 application”), the entire disclosure of each of which is incorporated by reference herein. FIG. 1B shows an example of one such CE 140, which includes or consists essentially of two NPN bipolar junction transistors (BJTs) 170, 171 and two resistors 180, 181, is electrically coupled in series with the string of LEDs 130, and may be located at one end of the string or anywhere mid-string. The value of resistor 181 determines the current between the two terminals of CE 140 identified in FIG. 1B as 190 and 191. Such a CE 140 acts like a two-terminal polarized device, allowing current to flow only in one direction and maintaining an essentially constant current, as determined by the value of resistor 181. Similar circuits using FETs may also be utilized, as described in the '807 and '027 applications. Versions of such circuits, as well as those incorporating additional features such as temperature compensation, are commercially available in single packages, for example the AL5802 manufactured by Diodes, Inc. In various versions, the current set resistor (e.g., resistor 181) may be internal or external to the package.
A disadvantage of these approaches is that the current is fixed, e.g., determined by a fixed resistance value of a current-set resistor, for example resistor 181 in the circuit of FIG. 1B. If the current set resistor is internal to the current control circuit package, then multiple packages are required to achieve different intensity values, with each package set to a different current. If the current set resistor is external to the package, a different resistor for each desired intensity level from the LEDs is required. In either case there is a requirement to stock a large number of parts (either current control packages or resistors) of different values in order to achieve a wide range of light output intensities from the LED illumination source. Furthermore, if finished goods are desired to be inventoried, for example to reduce lead time, then the number of different products required to be inventoried increases with the number of required light intensity values for each product, resulting in increased costs. In some cases, the required current value may not be known prior to the design of the lighting system, thus precluding manufacture of the complete system until the desired current level is determined, resulting in long delivery lead times and loss of economies of scale in producing large volumes of lighting systems in batch form.
In view of the foregoing, a need exists for systems and techniques enabling the low-cost, rapid design and manufacture of LED lighting systems having a wide range of light output intensities.